10:00 AM - 10:15 AM
[PCG22-05] Quantification of dangling OH bonds in amorphous H2O ice at low temperature using infrared spectrometry
Keywords:Icy dust grain, dangling OH bonds, infrared spectrometry
Cold and dense interstellar clouds contain silicate dust and molecular gases such as H2, which are precursors to stars and planets. Because of their cryogenic environment as low as 10 K, gases freeze onto dust surface to form amorphous ice mainly composed of H2O. Surface reactions on icy dust are important for rich chemistry in interstellar clouds [1]. Recently, the James Webb Space Telescope, high sensitivity infrared telescope in space launched in 2021, detected an absorption peak possibly assigned to “dangling OH bonds” for amorphous icy dust grains in an interstellar cloud [2]. It corresponds to the free OH stretching bands of two-coordinated (one H donor and one H acceptor; 2dOH) and three-coordinated (one H doner and two H acceptors; 3dOH) water molecules on the ice surface (Fig. 1). In Infrared spectra of amorphous ice vapor deposited at low temperature, they show two weak absorption peaks at 3720 cm-1 and 3696 cm-1, respectively [3]. The amount of dangling OH bonds provides a clue to the structure and reactions on the ice surface. However, the study on the number density of dangling OH bonds is still limited only at 90 K [4], preventing quantitative understanding on the surface of amorphous ice at low temperature in interstellar clouds.
In this study, we measured the number densities of 2dOH and 3dOH on amorphous ice at 20 K using infrared multiple angle incidence resolution spectrometry (IR-MAIRS). IR-MAIRS is a technique combining oblique incidence transmit measurements and multiple variation analysis to obtain pure out-of-plane (OP) and in-plane (IP) spectra (Fig. 2) [5]. Amorphous H2O ice sample was prepared by vapor deposition on Si substrate cooled at 20 K. After that, CO was deposited on ice surface as probe gas to quantify the number density of dangling OH bonds. The deposition rate of CO was 2.1×1012 cm-2 s-1.
Figure 3 shows IR-MAIRS spectra of amorphous ice at 20 K. Both 2dOH and 3dOH peaks appeared in OP and IP spectra and vanished after 40 min CO deposition (Fig. 3A). At the same time, CO stretching features newly appeared at 2139 cm-1 and 2152 cm-1 (Fig. 3B). From the higher peak assigned to CO interacting with dangling OH bonds (2dOH and 3dOH), the column density for 2dOH and 3dOH was deduced as 8.8×1014 cm-2. Furthermore, using the band strength (integrated absorption cross section) of 3dOH obtained in other experiments, the number density of 3dOH from the spectra in Fig 3A was calculated as 5.8×1014 cm-2. That for 2dOH was deduced as 8.8×1014 - 5.8×1014 = 3.0×1014 cm-2. When CO was deposited for 90 min, a sharp peak at 2142 cm-1 appeared in OP spectrum (Fig. 3C). This peak derives from the LO (longitudinal optic) mode indicating that CO covers the ice surface and that the multilayer adsorption has started. Assuming that the column density of CO molecules deposited for 80 min is equal to that of H2O molecules on the ice surface, the ratio of dangling OH bonds to surface molecules was estimated about 9 %.
References
[1] T. Hama & N. Watanabe, Chemical Reviews 113, 8783–8839 (2013).
[2] M. K. McClure et al., Nature astronomy 7, 431-443 (2023).
[3] V. Buch & J. P. Devlin, The Journal of Chemical Physics 94(5), 1, 4091-4092 (1991).
[4] T. Nagasawa et al., The Astrophysical Journal Letters 923, L3 (8pp) (2021).
[5] Nagasawa et al., J. Raman Spectrosc. 53(10), 1748-1772 (2022).
In this study, we measured the number densities of 2dOH and 3dOH on amorphous ice at 20 K using infrared multiple angle incidence resolution spectrometry (IR-MAIRS). IR-MAIRS is a technique combining oblique incidence transmit measurements and multiple variation analysis to obtain pure out-of-plane (OP) and in-plane (IP) spectra (Fig. 2) [5]. Amorphous H2O ice sample was prepared by vapor deposition on Si substrate cooled at 20 K. After that, CO was deposited on ice surface as probe gas to quantify the number density of dangling OH bonds. The deposition rate of CO was 2.1×1012 cm-2 s-1.
Figure 3 shows IR-MAIRS spectra of amorphous ice at 20 K. Both 2dOH and 3dOH peaks appeared in OP and IP spectra and vanished after 40 min CO deposition (Fig. 3A). At the same time, CO stretching features newly appeared at 2139 cm-1 and 2152 cm-1 (Fig. 3B). From the higher peak assigned to CO interacting with dangling OH bonds (2dOH and 3dOH), the column density for 2dOH and 3dOH was deduced as 8.8×1014 cm-2. Furthermore, using the band strength (integrated absorption cross section) of 3dOH obtained in other experiments, the number density of 3dOH from the spectra in Fig 3A was calculated as 5.8×1014 cm-2. That for 2dOH was deduced as 8.8×1014 - 5.8×1014 = 3.0×1014 cm-2. When CO was deposited for 90 min, a sharp peak at 2142 cm-1 appeared in OP spectrum (Fig. 3C). This peak derives from the LO (longitudinal optic) mode indicating that CO covers the ice surface and that the multilayer adsorption has started. Assuming that the column density of CO molecules deposited for 80 min is equal to that of H2O molecules on the ice surface, the ratio of dangling OH bonds to surface molecules was estimated about 9 %.
References
[1] T. Hama & N. Watanabe, Chemical Reviews 113, 8783–8839 (2013).
[2] M. K. McClure et al., Nature astronomy 7, 431-443 (2023).
[3] V. Buch & J. P. Devlin, The Journal of Chemical Physics 94(5), 1, 4091-4092 (1991).
[4] T. Nagasawa et al., The Astrophysical Journal Letters 923, L3 (8pp) (2021).
[5] Nagasawa et al., J. Raman Spectrosc. 53(10), 1748-1772 (2022).